1. Field of the Invention
The present invention relates to a light source device and a field-sequential-type projection device including the light source device.
2. Description of the Related Art
Recently, display-devices having a large screen have become rapidly widespread, and it is common to use these displays in conferences, presentations, training and so on.
As display-devices for the above-mentioned purpose, there are various devices such as liquid-crystal type displays and plasma type displays. The displays are suitably chosen according to the size of venue or turnout. In particular, projection devices (hereinafter referred to as “projector”) are the most prevalent large-screen display-devices because of their relative inexpensiveness and portability (i.e. they are light, compact and easy to carry).
Under such circumstances, recently, situations calling for communication have been increasing, and for example, conferences and meetings using a projector in a small conference room or a partitioned meeting space have been held frequently in many offices.
Moreover, situations arise where a meeting needs to be held at short notice in, for example, an unused space such as a passage if no meeting room is available, and information needs to be projected on to a wall and so on with a projector.
As a representative example of the projector, there is a field-sequential type projector represented by a DLP (Digital Light Processing) type projector. In the DLP type projectors, a display element such as a Digital Micromirror Device (DMD) achieves a gradation expression with modulation of a source light by high-speed switching of the angle of micro-mirrors of each pixel arranged in a matrix corresponding to an image data by use of PWM (Pulse Width Modulation) driving.
There are advantages in that these DLP type projectors are capable of being easily downsized and displaying each color, R(Red), G(Green) and B(Blue) sequentially by switching in time since the DMD is capable of high-speed motion.
On the other hand, conventionally, a main light source used for the projector is a high-intensity discharge lamp such as an extra high pressure mercury lamp. However, recently, products using the following as a light source have been launched onto the market: a solid-state light-emitting element such as an LED and an LD of red, green and blue or a fluorescent body emitting light of a different wavelength band from the light emitted from the solid-state light-emitting element as excitation light.
The solid-state light-emitting element has the advantage of being convenient in that there is no need for a cooling period after power-OFF like with a conventional lamp and it emits light of proper luminance immediately after power-ON.
Furthermore, the solid-state light-emitting element is considered to be environmentally friendly because it does not use mercury.
However, the amount of luminescence of the solid-state light-emitting element and the fluorescent body vary depending on the temperature, and in general, decrease with an increase in temperature. That is to say, solid-state light-emitting elements decrease the amount of luminescence from the start of energization by heat generation of themselves even if a constant electric current is supplied as a common driving method, and the amount of luminescence becomes stable when the solid-state light-emitting elements become thermally-stable.
The fluorescent body, which produces fluorescence by irradiation of the excitation light, also decreases the amount of luminescence by heat generation of itself even if constant excitation light is irradiated, and the amount of luminescence becomes stable when the fluorescent body is thermally stable.
Thus, when the solid-state light-emitting element or the fluorescent body is applied to the DLP type projector mentioned above, there has been a problem in that a gradation expression cannot be performed properly due to the amount of luminescence varying with driving by the common method as seen above even though the amount of luminescence of the light source must be constant for achieving the gradation expression by PWM driving.
Therefore, for example, Japanese Patent Application Publication No. 2006-349731 (Patent Literature 1) and Japanese Patent Application Publication No. 2010-085725 (Patent Literature 2) seek to solve the problem mentioned above.
In Patent Literature 1, a device to finely control the amount of current passing through the light-emitting element to keep the amount of luminescence of the solid-state light-emitting element constant is disclosed.
Further, in Patent Literature 2, a device to measure a level of an actual gradation relative to an original gradation signal and to convert the original gradation signal for adjusting to the actual gradation is disclosed.